Generally, the mammalian circulatory system is comprised of a heart, which acts as a pump, and a system of blood vessels which transport the blood to various points in the body. Due to the force exerted by the flowing blood on the blood vessel the blood vessels may develop a variety of vascular disabilities or dysfunctions. One common vascular dysfunction known as an aneurysm results from the abnormal widening of the blood vessel. Typically, vascular aneurysms are formed as a result of the weakening of the wall of a blood vessel and subsequent ballooning of the vessel wall. As shown in FIG. 1, the aneurysm 10 often comprises a narrow neck portion 12 which is in communication with the blood vessel 14 and a dome portion 16 in communication with the neck portion 12. As shown in FIG. 1 the neck portion 12 and the dome portion 16 form a cavity 18. Aneurysms have been known to form in a plurality of location though the body, including, for example, the brain, the abdomen, and throughout the circulatory system.
In response, several surgical techniques for treating aneurysms have been developed. Initially, an aneurysmectomy was required to repair the dysfunctional tissue. The aneurysmectomy procedure requires the surgeon to gain access to the aneurysm, excise the aneurysm, and replace the void with a prosthetic graft. Because this is a major surgical undertaking, the mortality rate of the procedure is relatively high. Commonly, the aneurysmectomy procedure is unavailable to patients with severe coronary or cerebral arteriosclerosis, severe restrictive pulmonary disease, and significant renal disease or other complicating factors. An alternate method of treating cerebral aneurysms called ‘microsurgical clipping’ requires the placement of a metallic clip across the neck of the aneurysm, thereby excluding the aneurysm from the blood flow.
In response to the shortcomings of the aneurysmectomy and the microsurgical clipping procedures, less invasive methods of treatment have been developed. Commonly, these procedures require the formation of an artificial vaso-occlusion, which is obtained by implanting a number of devices or suitable materials into the cavity 18 of the aneurysm, thereby resulting in a decrease in the flow of blood into the aneurysm. The reduced flow results in hemostasis and the formation of a clot. Generally, this procedure requires the surgeon to advance a micro-catheter to a location inside the aneurysm and deposit a biologically-compatible vaso-occlusive material or device therein. Typical vaso-occlusive devices and materials include platinum micro-coils, hog hair, microfibrillar collagen, various polymeric agents, material suspensions, and other space filling materials.
FIG. 2 shows an aneurysm 10 formed on a blood vessel 14, the aneurysm 10 having a vaso-occlusive device 20 positioned within the aneurysm dome 18. A disadvantage of filling an aneurysm with devices is that the vaso-occlusive mass may impinge on nerves or other biological structures, thereby resulting in adverse biological symptoms. For example, the impingement of the vaso-occlusive device 20 on structures or nerves within the brain, commonly known as ‘mass effect’, may result in adverse neurological symptoms. Another problem associated with vaso-occlusive devices is maintaining the device within the aneurysm. Blood flow through an otherwise functional blood vessel may be compromised should the device migrate from the aneurysm during or following implantation, thereby possibly resulting in a vascular embolism. Yet another problem associated with certain vaso-occlusive devices, such as coils, is that the coils may migrate out of aneurysms having wide necks into the parent vessel. Thus, only aneurysms having certain dome to neck ratios can be treated in this fashion.
An alternate method of repairing an aneurysm has been developed which requires the implantation of a mechanical support device within the blood vessel near the neck portion of the aneurysm. Generally, these mechanical support devices, commonly referred to as “stents”, comprise deployable mechanical support structures capable of delivery to a situs within the blood vessel through catheters. In addition to providing mechanical support to the dysfunctional vessel wall, the stent may include a mechanical structure which seeks to restrict the blood flow though the portion of the blood vessel proximate the aneurysm, thereby reducing or eliminating the aneurysm. The stent may also be useful in preventing coils from migrating out of the aneurysm. Exemplary mechanical structures capable of restricting blood flow to an aneurysm include meshes or fenestrated structures which are positioned near an aneurysm 10 and restrict the flow of blood thereto.
FIG. 3 shows a stent 22 positioned in a blood vessel 14 proximal to an aneurysm 10. While a stent may provide adequate mechanical support to the blood vessel, these devices have demonstrated limited effectiveness in limiting blood flow to the aneurysm. As such, the aneurysm typically remains intact and may increase in size. In response, stents may be covered with various coatings designed to limit blood flow to the aneurysm. These coatings typically include biologically compatible polymers, films, and fabrics. However, the application of these coatings to the stents increases the cross-sectional diameter of the device, thereby resulting in a high profile stent-graft. As a result, the blood flow through the blood vessel is reduced by the presence of a high profile stent-graft. In addition, device profile is a significant problem for the treatment of cerebral aneurysms due to the small size of the cerebral blood vessels, therefore requiring the device to be deliverable to the aneurysm through a micro-catheter. As such, high profile stent-grafts are typically not used in the treatment of cerebral aneurysms.
There are additional limitations in the use of conventional stents to treat cerebral aneurysms. Since stents have a cylindrical shape, it is less useful for treating an aneurysm that is formed at a bifurcation of an artery or in arteries with complex geometries. Also, self-expanding stents can be difficult to deliver because, when collapsed, they will exert a radial force on the delivery sheath that is proportional to its length. The user must overcome this radial force to deliver the stent. As the delivery sheath is retracted against this force, energy is stored in the delivery system and released as the stent is deployed, causing considerable movement in the system during deployment. This issue is usually addressed by utilizing a stent that is longer than the aneurysm neck so that accuracy of delivery is less crucial. However, this also tends to increase the radial force holding the stent in the delivery system which only exacerbates the problem of stored energy, thus creating a viscous cycle. Balloon expandable stents can mitigate this problem, but the balloon makes the device stiffer and more difficult to navigate tortuous cerebral anatomy.
Thus, there is presently an ongoing need for a device and method for effectively treating aneurysms without significantly affecting blood flow through the blood vessel.
There is also an ongoing need for a device that can be used in conjunction with or in lieu of coils that can occlude an aneurysm without adversely affecting blood flow through the vessel.